This grant application is made to continue work currently in progress in order to facilitate a move to a new institution. The research will test the hypothesis that neuronal activity is required to maintain structural integrity of synapses in the auditory system. Previous models used to study this hypothesis included both sound deprivation of adult animals and sequential studies of the rat anteroventral cochlear nucleus (AVCN) during development. The proposed research will investigate the effects of noise-induced damage of auditory hair cells on second order synapses in nucleus magnocellularis of the chicken using the freeze-fracture technique. Noise-induced hearing loss is considered a good model to study the effect of decreased activity in these synapses because, although the spiral ganglion cells are histologically normal after severe damaged hair cells is eliminated. (2,3) The specific aims of the research are: 1. To examine primary afferent synapses in chick nucleus magnocellularis with the freeze-fracture technique after noise-induced temporary threshold shift (TTS) in the hatching chick. 2. To examine primary afferent synapses in chick nucleus magnocellularis with the freeze-fracture technique after noise-induced permanent threshold shift (PTS) in the hatching chick. The research plan will involve examination of pre-and postsynaptic membranes of the spherical cells of nucleus magnocellularis at increasing time intervals after severe noise exposure in young chickens. The basilar papillae of the same animals will be evaluated by light microscopic hair cell counts and by scanning electron microscopy. Ultrastructural changes in nucleus magnocellularis will be correlated with changes in the basilar papillae.